Waveguide coupling



Sept. 2, 1958 l J. MuNusHlAN WAVEGUIDE COUPLING 2 Sheets-Sheet 1 Filed Feb. 4. 1955 Sept. 2, 1958 J. MuNusHxAN WAVEGUIDE couPLING 2 Sheets-Sheet 2 Filed Feb. 4. 1955 lhelix.

waveguide. :a helical slow-wave structure, or to extract wave energy WAVEGUIDE CoUPLlNG Jack Munushian, Pasadena, Calif., assigner to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application February 4, 1955, Serial No. 486,170 4 Claims. (Cl. S33-64) This invention relates to waveguide transition structures and more particularly to an electromagnetic coupling between a rectangular waveguide and a periodic slow-wave structure providing a broadband impedance match.

A periodic slow-wave structure such as, for example, a conductive helix is frequently employed in traveling-wave tubes. An electron gun is then utilized to project an electron stream in an interacting relationship with an electromagnetic wave which may be propagated along the It is conventional practice to feed the slow-wave .structure of a traveling-wave tube with a rectangular In order to launch a traveling wave along propagated therealong, antenna means must be connected :between the slow-wave structure and the rectangular waveguide.

It is at present the practice to extend or expand the last few turns on a traveling-wave tube helix to form a dipole antenna, one end of which is grounded to the waveguide wall or an electrical equivalent thereof. A traveling-wave tube has an unusually large operating frequency range. 'This characteristic makes its use very desirable in a number of applications. However, the radiation resistance of a dipole is relatively frequency sensitive whereby the broadband operating characteristic of a traveling-Wave tube is somewhat restricted. The transition from a helical to a linear conductor is not a gradual physical transition. This, in turn, further limits the operating frequency range of a traveling-Wave tube.

It is, therefore, an object of the invention to provide a broadband impedance match between a rectangular waveguide and a periodic slow-wave structure.

It is another object of the invention to provide an electromagnetic coupling between a rectangular waveguide and the slow-wave structure of a traveling-wave tube which provides an impedance match over a broad frequency range.

In accordance with the invention, a slow-wave structure such as used in a traveling-wave tube and having generally a cylindrical form is disposed transversely through an aperture in a rectangular waveguide, one end of the slow-wave structure being electrically connected to the opposite inside wall of the waveguide or an electrical equivalent thereof. The diameter of the slow-wave structure is then decreased to provide a cone leaving the ends of the slow-wave structure flared radially outward.

A relatively high-power slow-wave structure for microwave tubes is disclosed in a copending application, Serial No. 450,987, entitled High-Power Microwave Tube, led August 19, 1954, by Charles K. Birdsall. One embodiment of the present invention is particularly useful for coupling wave energy between that slow-wave structure and a rectangular waveguide.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects `and States Patent ice advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which a number of embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a sectional view of a traveling-wave tube employing the broadband impedance match of the present invention;

Fig. 2 is an enlarged broken-away sectional view of a portion of the traveling-wave tube of Fig. l;

Fig. 3 is an isometric projection of the slow-wave structure shown in Figs. l and 2;

Fig. 4 is a schematic View illustrating the electrical equivalent of the slow-wave structure shown in Figs. l to 3; and

Fig. 5 is a broken-away sectional View of another embodiment of the coupling of the invention providing a broadband impedance match.

Referring to the drawings, Fig. l shows a traveling-wave tube 10 comprising a conductive evacuated envelope 12, at the left end of which an electron gun 14 is shown for producing an electron stream along a predetermined path 16 within the envelope 12.

Shown disposed concentrically about the envelope 12 is a focusing solenoid 18 which is employed to constrain or to conne the stream electrons as they are directed along the path 16. The solenoid 1S is provided with a source of direct current such as a battery Ztl.

Electron gun 14 comprises a cathode 24 which is provided with a filament 22. A focusing electrode 26 is disposed about the path 16 to the right of the catho-de 24 and an accelerating anode 28 is disposed about the path 16 adjacent focusing electrode 26. Focusing electrode 26 may have a frusto-conical configuration with an internal surface of revolution disposed at an angle of 671/2 degrees from its axis of symmetry. The cathode 24 is maintained at a potential'which may be a few thousand volts negative with respect to ground by means of an accelerating source of potential 30. Filament 22 is provided with a filament source of potential 32, the negative terminal of which is connected through a stem lead 34 to the negative terminal of filament 22 and to the cathode 24, the stem lead being insulated from the conducting envelope 12. The accelerating anode 28 is maintained at a few hundred volts positive with respect to the cathode 24 by means of a source of potential 29 having its positive terminal grounded.

An internal input waveguide segment 36 is disposed within envelope 12 and is insulated therefrom. The input waveguide segment has registering apertures 3S and 40 disposed in opposite walls 42 and 44 of a waveguide portion 46 which extends transversely to the path 16. The apertures 3S and 4% thus permit the electron stream to be projected along the path 16 through the transverse portion 46 of the internal input waveguide 36.

A conductive slow-wave structure 48 having flaring end portions Sii and 52 is mechanically and electrically connected to the internal surface of waveguide walls 42 and to the internal surface of waveguide wall 54 which forms part of an internal output waveguide segment 56. The slow-Wave structure 4S thus comprises a plurality of interconnected conductive rings which decrease progressively in diameter in the axial direction along the slowwave structure 43 from the internal surface of the waveguide wall 42. Internal output waveguide 56 is electrically insulated from the envelope 12-and is provided with registering apertures 59 and 61 corresponding to the apertures 38 and 40 in internal input waveguide 36. A dielectric apertured disc 58 is employed to maintain the internal waveguides 36 and 56 in position andvto support external input and output waveguides 60'and 62, respectively, and a collector electrode 64. A high vacuum is maintainedl within the envelope 12 which. issided-hy two, micawindows 66and 68 whichare disposedtransversely wthin the externalinput and outputv waveguides. Q-.and 6,2, respectively. Y

Collector 64 is maintained a few hundredvoltspositivewithrespect to waveguide portion 56 by means of. a. s ource of potential 65 in order to prevent secondary emission to the waveguide portion 56.

An enlarged broken-away sectionall view of slow-,wave

structure48 and the transverse portion 46 of the internal Y Y input waveguide 36 is shown in Fig. 2. The transverse waveguide portion 46 has the Vaperture 38 in the left wall 42 and the second aperture 4l)V in the rightwallA 44. The aperture 33 need be no larger than the path 16v shown in Fig. l, but should be smaller than the maximum-diameter of the slow-wave structure 48 in order. that a simple electrical connection maybe provided between the transversewaveguide portion 46 vand the slow-wave structure 48. The endsrof slow-wave structure 48 are accordingly brazed to the internal surfaces of the wave-guide walls 42 and 54 of the waveguide portions 46 and 56, respectively. The width of the transverse waveguide portion 46 should generally be equal to the axial. distance along right wall 158 thereof. to and concentrically within theenvelope 154 is a matching ferrule 160 to which a helix 162 having a tapering spiral is electrically connected.. It will be understood that a glass envelope may be analogously employed with a slow-wave structure similar to or the same as the slow- Wave structure 48. The, helix.16,2 may have a constant pitch, i. e., the distance' fronrturn to turn may be the same as is the.. spacing of the conductive rings of the slow-wave struetureASf. The rightn end of helix 1.62 may be .similarly constructed. The insideu diameter of f errule 160 may be as small as desired depending upon` the size of the beam which is employed to interact with electromgnetic waves propagatedalongitheVl helix 162.' The axial length of the ferrule 160.. and the sleeve 152 zof wave guide 150 may be'chosen to produce =a vertical Vshorting plane at the left internal surface 164 of the wave- Y guide 15o.

the slow-wave structure 48 from the end brazed to Waves guide wall 42 to the portion of the slow-wave structure 48 having a uniform diameter. For example, for the slow-wave structure 48, the right wall'44 of the transverse waveguide portion 46 is positioned abouta conducof the slow-wave structure 48 begins. Empirioally it has been found that the aperture 40. in the'wall 44 of the tive ring 120 where the taper on the left hand end flare .Y

diameterof the slow-wave structure 48. A plurality of axiallyconductive segments 122 is shown connecting alternate adjacent pairs of conductive rings 124. The slow- Y wave structure 48V and the Vstructure o f bilar conn-awound helices is disclosed and claimed in the previously mentioned Birdsall application.

It has. been found that an optimum broadband match will generally occur if the slow-wave structure 48 and the transverse waveguide portion 46 are constructed in the manner shown, i. e. all of the conductive segments 122 lie in a plane substantially parallel tothe direction of energy propagation in the waveguide portion 46. The distance between conductive rings may be substantially constant throughout the slow-wave structure 48( The transverse waveguide segment 46 has a shorting termination 47, the position of which may be chosen for an optimum broadband match for a given taper of the flaring endV 5i) of the slow-wave structure 48. TheY aring end 52 of slow-wave structure 48 is constructed in the same manner as ilaring end 50. The slow-wave structure 48 is the electrical equivalent of uniiil-ar contrawound conductive helices. Fig. 3 is anisometric broken sectional view of slow-wave structure 48 and Fig. 4 shows two wires 146 and 142 which are in electrical contact twice per turn. The wires 149 and 142 thus simulate the electrical equivalent of the slow-wave structure 48, and the contacts of "the wires 140 and 142 at points 144, 146 and148 are electrically equivalent to axially conductive segments 130, 132, and 134, respectively, in the portion of slow-wave structure 48 shown in Fig. 3. Y l

Fig. 5 illustrates how the broadband impedance match of the present invention may be employed in a traveling- -wave tube having a glass envelope.4 A transverse waveguide segment 1750 is thus illustrated ,in Fig. 5 having an axially extending sleeve 152 which is disposed about an elongated evacuated glass envelope 154. The waveguide segment 150. is provided with Van aperture 156 in the cal `slow-wave structure oran equivalent thereof. A y

taperedlow-wave structure having end flares connected to the internal surfaces of a rectangular waveguide or an electrical equivalent thereof thus acts as a conical helix radiating toward a ground plane which is one of the internalsurfaces of the rectangular waveguide. It has been found.V that, this type ofradiation operates over an unusually broadA band of frequencies. The broadband impedance match ofthe present invention therefore is very well adaptedfor'such-an application. Because a single tapered conical Vhelix radiates 'circularly polarized waves in -a rectangular waveguide and tapered contrawound conical helices radiate linearly polarizedwaves, the broadband impedance Ymatch provided by the coupling of the present invention may be found to be somewhat more useful in matching` tothe slow-wave'structure 48 or to other` such structures, such as, for example, the bilar contrawound helical slow-wave structure described in the aforementionedv Birdsall application. The broadband coupling ofthe invention still has utility'in matching to a unilar helix because of Vits broadband'characteristics inherent inthe gradual physical transition which may be made from a unilar helix to a helix having =a tapering spiralform. Y Y

` A representative example ofa good match from a rectangular waveguide toa unilar contrawound helical slow-wave structure, such as the slow-wave structure 48,

is given in terms of a standing Wave ratio over a predetermined frequency band. To thus obtain a standing wave ratiov of lessV than '1.2 over a twenty percent fre-- quency band width isV readily possible;

What is claimed is:

1. An electromagneticcoupling providing a broadband impedance. match-between a rectangular waveguide and a slow-wave structure comprising a'rectangular waveguide having a rst aperture in one wall and a second registering and concentric aperture inthe opposite Wall, vand a conductive helix having an outward radially haring end portion formingl a helical., conical antenna disposed through said first aperture ofsaid one waveguide wall and connected to sid'opposite wall ofV said waveguide, said second aperture, in said opposite waveguideV wall being smaller in diameter than themaximum diameter of said helix.

2. An electromagnetic coupling providing a broadband impedance match between Ya rectangular waveguide and a slow-wave structure comprisinga Vrectangular waveguide having at least one aperture in one wallthereof, and a slow-wave structure including a plurality of conductive rings, alternate adjacent pairs of said rings being connected alternately at diametrically opposed points, the diameter of said rings progressively decreasing for a predetermined distane from atleast one end of said slow- Shown positioned contriguouslyV axial distance between said rings being substantially constant over the complete length of said slow-wave structure, the diameter of said conductive rings being approximately equal to two-thirds of the diameter of the aperture of said waveguide wall, the end portion of said slow-wave structure forming said conical type of antenna being disposed through the aperture of said waveguide wall and being electrically connected to the wall of said waveguide opposite the wall dening said aperture.

3. An electromagnetic coupling providing a broadband impedance match between a rectangular waveguide and a slow-wave structure comprising a rectangular waveguide having an aperture in at least one wall thereof, andV a conductive helix having a radially progressively decreasing diameter for a predetermined distance from at least one end thereof to form a helical, conical antenna along said distance, the pitch of said helix being substantially constant over its entire length and the diameter of said helix being substantially equal to one-third of the diameter of the aperture in said waveguide wall, the end portion of said helix having said decreasing diameter being disposed through the aperture of said waveguide wall and being electrically connected to the wall of said waveguide opposite the wall dening said aperture.

4. An electromagnetic coupling providing a broadband impedance match between a rectangular waveguide and a slow-wave structure comprising a rectangular waveguide having an aperture in at least one wall thereof, and a slow-wave structure including a plurality of conductive rings having at least one axially conductive segment connecting alternate adjacent pairs of said rings at diametrically opposed points, said rings having progressively decreasing diameters for a predetermined distance from one end of said slow-wave structure to form a conical type of antenna, said end of said slowwave structure forming said conical type of antenna extending through the aperture in said waveguide wall and being electrically connected to the wall of said waveguide opposite the wall defining said aperture.

References Cited in the le of this patent UNITED STATES PATENTS 1,945,544 Conklin Feb. 6, 1934 2,530,171 Okress Nov. 14, 1950 2,624,859 Smullirr Jan. 6, 1953 2,637,775 Lund May 5, 1953 2,712,614 Field July 5, 1955 2,730,647 Pierce Ian. 10, 1956 2,768,322 Fletcher Oct. 23, 1956 2,800,605 Marchese July 23, 1957 

